Boeing said in a statement that the Chinese airline also agreed to buy seven Boeing 757-200 passenger jets, which it will convert into freighters, as part of a deal signed on Tuesday. Financial terms weren't disclosed.

The U.S. aerospace firm has yet to launch a formal program to convert older 737-800 passenger jets into freighters. But China Postal, which already operates a fleet of over 30 Boeing aircraft according to its website, joins Hangzhou-based YTO Airlines as a launch customer for the converted aircraft, making a full conversion program launch more likely.

The jet maker has been gauging customer interest in the converted aircraft as the current generation of 737 passenger planes is to be replaced by the upgraded 737 Max passenger aircraft later this decade.